Thermally insulated shell mold and method for making same



1968 w. M. LENAHAN ETAL 3,

THERMALLY INSULATED SHELL MOLD AND I METHOD FOR MAKING SAME Filed Sept.4, 1964 2 Sheets-Sheet l INVENTORS WILLIAM M. LENAHAN ARTHUR G. WATTS mgd i ffk ATTORNEYS Feb. 6, 1968 w. M. LENAHAN ETAL 3,367,393

THERMALLY INSULATED SHELL MOLD AND 1 METHOD FOR MAKING SAME Filed Sept.4, 1964 2 Sheets-Sheet 2 INVENTORS WILLIAM M. LENAHAN ARTHUR G. WATTSATTORNEYS United States Patent ()fifice 3,367,393 Patented Feb. 6, 19683,367,393 THERMALLY HNSULATED SHELL MOLD AND METHOD FOR MAKING SAMEWilliam M. Lenahan, Denville, and Arthur G. Watts, Morris Plains, NJ.(both Howe Sound (30., 500 5th Ave., New York, N.Y. 10036) Filed Sept.4, 1964, Ser. No. 394,454 17 Claims. (Cl. 16434) This invention relatesto thermally insulated refractory shell molds and, more particularly, toa shell mold, and its method of manufacture, characterized by amultiplicity of voids within its Wall which reduce the cooling rate ofthe mold and the casting therewithin.

In the manufacture of accurate metal castings, it is customary to make apattern of the desired casting in wax or a similar fusible material andthen to form a refractory mold by dip-coating the pattern a number oftimes in a liquid suspension of a finely divided refractory whichhardens in layers on the pattern. When a refractory shell has thus beenbuilt up, it is only necessary to eliminate the pattern from within itin order to form an accurate mold of the article to be cast. At the timethe mold is to be used in a casting operation, the practice is topreheat it to casting temperature so that the molten metal can best fillthe mold cavity without solidifying prematurely.

To avoid defects in a casting such as shrinkage and the formation ofvoids, the cast metal must feed into and fill out all sections of themold cavity. This cannot be achieved if some portions of the castingsolidify too fast, and therefore the cooling rate of the casting must becontrolled. For this reason, it has been the custom in the precisioncasting industry to encase the outside of the mold in a dry backupmaterial or an asbestos wrapping which serves as a thermal insulator tolessen the cooling rate of the shell.

A primary object of the present invention is to eliminate the necessityfor external insulation of this sort by undertaking unique measuresduring the manufacture of the shell mold to form a cellular wallstructure of substantially reduced thermal conductivity.

Broadly stated, the method of the invention is applicable to theconventional manufacture of a refractory mold wherein a multi-layeredshell is formed by successively coating and hardening slurriescomprising a binder and a fine refractory on a pattern and thereaftereliminating the pattern. In accordance with the method, an evendistribution of a particulate thermally decomposable material isembedded in at least one of the slurry coats subsequent to the firstbefore it is hardened. The shell is subsequently heated above thedecomposition temperature of the material to eliminate the material andleave voids distributed throughout the shell wall. The thermally insulated refractory shell mold produced by this method comprises the usualplurality of layers of bonded refractory material. It is characterizedby the fact that at least one layer excluding the inner layer defines atleast part of the multiplicity of voids which reduce the cooling rate ofthe mold. At least the inner layer is free of such voids.

When a refractory mold is manufactured in accordance with the invention,its thermal insulating means is incorporated in situ'within the cellularwall of the shell itself. The multiplicity of voids reduces outward heatflow from the inner casting surface of the mold to the point where itbecomes unnecessary to undertake separate steps toward insulating themold, such as surrounding it with a dry backup material or a wrapping ofasbestos. The mold itself is thus capable of maintaining its desiredtemperature between the time it leaves the pre-heat furnace and isfilled with the molten metal in the casting process.

Even very thin sections filled out satisfactorily during casting sinceno quick chilling occurs as the molten metal is fed into the moldcavity. Also and due to the insulatin g efiect of the mold made inaccordance with the invention, rapid cooling of the metal in the mold isprevented and a sound casting free of shrinkage and from defects isproduced. All of this is accomplished without additional externalinsulation for the mold because the voids in its wall are sufiicient forreducing the cooling rate both of the mold and of the castingtherewithin.

Preferred embodiments of the method and product of the invention aredescribed hereinbelow with reference to the accompanying drawing,wherein FIG. 1 is a simplified illustration of a refractory shell moldbuilt up on a pattern;

FIG. 2 is an enlarged section taken along the line 22 of FIG. 1illustrating a shell formed on the pattern in accordance with oneembodiment of the invention;

FIG. 3 is a similar section of the FIG. 2 shell when it is ready forcasting;

FIG. 4 is a similar section of a shell formed on a pattern in accordancewith another embodiment of the invention; and

FIG. 5 is a similar section of the shell of FIG. 4 when it is ready forcasting.

Referring first to FIGS. 1 to 3, an accurate pattern 10 of the articleto be cast is first made by any common procedure, preferably using afusible material such as wax, a thermoplastic composition, or the like.In forming a shell mold 11 about the pattern 10, a number of coats of arefractory composition are applied to the pattern. The process known asdip-coating is a particularly satisfactory technique for applying thecoats, involving the steps of immersing the pattern in a vesselcontaining an adequate depth of the coating composition and then liftingit out and holding it out for a brief time over the vessel while theexcess composition drains off.

The composition used for each dip-coating is a slurry of controlledviscosity comprising, among other things, a liquid suspension of finelydivided refractory particles and a binder. The finely divided refractorymay be zirconium silicate, zirconium oxide, aluminum oxide, silica, orother well known materials usually having a particle size at least aboutminus 270-mesh. Typical slurries in which the refractory material issuspended may have a binder of hydrolyzed ethyl silicate or isopropylsilicate, in which case hardening would be carried out by gelling theslurry dip-coat, perhaps in an atmosphere of ammonia gas or in air or bymeans of chemical gelling agents. Alternatively, the slurries maycomprise a suspension of the refractory material in an aqueous solcontaining colloidally dispersed silica, in which case hardening isusually achieved as a result of evaporation of water from the solvehicle so that the colloidal constituents of the sol coagulate and bondthe finely divided refractory particles together.

As suggested by the dotted lines in FIGS. 2 and 3, the mold 11 developedon the pattern 10 is actually formed of a plurality of layers resultingfrom the respective dipcoating operations, nine such layers 11a to 111'aggregating from about one-eighth to one-half of an inch thickness areshown in this embodiment. Since the slurry used to form the first layer11a defines the inner or casting furnace of the finished mold, it shouldhave an especially smooth surface. The refractory material suspended inthe slurry of the first dip-coating step should therefore beparticularly fine, for example, about 90% minus 325- mesh. The usualpractice is to sand each coating by sprinkling it while still wet with arelatively coarse refractory, such as fused alumina or grog fine enoughto pass a 60-mesh screen but not over about 20% minus 200-mesh. Thesecoarser particles embed themselves in the coating and provide a roughsurface to which the next succeeding coating can easily become bonded.Such sanding is preferably carried out in the practice of thisinvention, though it is modified to some extent in the second embodimentdescribed hereinafter. No attempt has been made to'illustrate theseinter-layer refractory particles in the drawing.

In accordance with the invention, an even distribution of a particulatethermally decomposable material is embedded in at least one of theslurry coats subsequent to the first before it is hardened. The organicparticulate material may be plastic, wood chips, cork, sawdust, cornchips, or any other material which decomposes substantially completelyat temperatures below the pre-heat temperature to which the mold issubjected before casting, for example, about 1800 F. In the embodimentof FIGS. 2 and 3, wood chips are used as the organic particulatematerial. These are particles of chipped wood of fairly regular size,more or less cubicle and up to about onefourth of an inch or so on aside. Wood chips do not have an apparent overall fineness substantiallydifferent from coarse sawdust but they are more regular in size. Itshould also be noted that wood chips are a commercial product which isreadily available.

After the layers 11a and 11b have each been applied by dip-coating andthen sanded and hardened, the third layer 11c is applied and while it isstill wet it is sprinkled with an even distribution of wood chips 12a.The wood chips 12a embed themselves partially into the surface of thelayer 110 and are retained on it when the layer is hardened. In mostcases it is contemplated that the wood chips 12a will be the soleparticulate material applied to the layer 110 but it is not excludedthat a certain amount of the refractory sanding material may be appliedwith them if desired. After the layer 110 is hardened with the woodchips 12a attached to it, the layer 11d is applied over the Wood chipsto cover them and embed them within the shell. Once the layer 11d hasbeen hardened and sanded in the usual manner, the layer He is applied,sprinkled with more wood chips 12b and hardened. This is repeated withthe layers 11 and 11h covering the layers 12b and 12c respectively ofwood chips, while the outer layer 111' is of a conventional nature freeof wood chips.

When the last layer 111' has been hardened, the mold containing thepattern is ready for the next operation which is elimination of thepattern material. This may be done by inverting the mold so that itssprue is directed downwardly and heating it above the fnshiontemperature of the pattern 10 so that the pattern material melts anddrains off. Various techniques are known for eliminating the pattern andthe particular choice is of no special significance in the practice ofthis invention.

At this stage of the process when the pattern has been eliminated, theshell stands alone in its green state comprising nine bonded layers ofrefractory material with the inner two and outermost layers being of aconventional nature and the alternate adjoining layers Ila-11d, 11e- 11band 11g11h defining between them respective layers 12a, 12b and 120 ofeven y distributed wood chip particles. In accordance with theinvention, the shell is to be heated above the decomposition temperatureof the thermally decomposable material (i.e., the wood chips) toeliminate the material and leave voids distributed throughout the shellwall. The heating may be a step unto itself, but it is often preferredto leave the wood chips Within the wall of the green mold until the timecomes to pre-heat the mold to casting temperature. This is done byintroducing the mold into a hot oven where it is heated in the usualmanner to about 1800 F., at which temperature all traces of wood chipsin the wall of the mold undergo combustion. As a result, a multiplicityof evenly distributed air-filled voids 13a, 13b and 13c are left definedbetween the alternate adjoining layers 11c11d, 11e- 111 and 11g11h, theremainder of the layers being free of such voids. When the mold 11 isremoved from the pre-heating oven, its inner casting surface defined bythe first layer 11a cools very slowly compared to that of a conventionalmold because the multiplicity of voids within the portions of the shellWall closer to its exterior serve as effective insulating meanssubstantially retarding outward heat transfer. Because the castingsurface of the mold remains at a satisfactory casting temperature for agreatly protracted period, no difliculty is involved in carrying out thecasting process without chilling the molten metal as it fills the moldcavity. Even very thin sections can therefore be cast with precision,and there is no danger that the mold will be inadequately or improperlyfilled with the molten metal or that X-ray shrinkage will be exhibitedin the completed casting. All this is made possible solely because ofthe multiplicity of voids defined in the shell wall and there is no needto encase the mold in a dry backup material or an asbestos wrapping asthe practice has been heretofore.

Turning now to the embodiment of the invention with which FIGS. 4 and 5are concerned, a pattern 15 is given a series of slurry dip-coats toform a mold 16 comprising layers 17a to 17i, all steps being the same asthose described in the previous embodiment but for the exceptions notedbelow. Instead of sprinkling or dusting certain of the layers with aparticulate thermally decomposable material, the material is dispersedprior to dipcoating throughout the slurries used for the alternatelayers 17d, 17) and 17h. The slurries used for these three dipcoats havethe particulate thermally decomposable material mixed directly withinthem along with the finely divided refractory and the binder so that thedecomposable material is automatically embedded as shown at 18a, 18b andin the respective dip-coats as they are applied. Wood chips are not asdesirable for this embodiment of the invention as in the embodimentdiscussed previously because they can have an adverse effect on theviscosity of the slurries. Sawdust serves well for admixture directlyinto the slurries as do corn chips, cork, and certain thermallydecomposable plastics.

Each of the layers 17a to 171' may be sanded with a relatively coarserefractory material in the conventional manner, including the layers17d, 17f and 1711 which contain the decomposable particles. When themold 16 containing the pattern 15 is completed, there is an evendistribution of the various decomposable particles 18a, 18b and 180 Withsubstantially every one of the particles embedded wholly within one ofthe respective dip-coat layers 17d, 17) and 1711. The pattern 15 iseliminated as before and the mold 16 is heated to a temperature abovethe decomposition temperature of the decomposable particles preferablyat the time the mold is pre-heated for casting. As shown in FIG. 5, thisburns out the particles and leaves the alternate layers 17d, 17f and 17heach defining a multiplicity of evenly distributed air-filled voids 19a,19b and respectively which reduce the cooling rate of the mold. Theinner three layers 17a, 17b and 170, the outermost layer 17i, and theremaining layers 17e and 17g are free of such voids. Prior to and duringcasting, the insulating effect of these voids prevents chilling of themolten metal to the same extent as in the previous embodiment.

We claim:

1. In the manufacture of a refractory mold wherein a multi-layered shellis formed by successively coating and hardening slurries comprising abinder and a fine refractory on a pattern and thereafter eliminating thepattern, a method of thermally insulating the shell which comprises (a)embedding aneven distribution of a particulate thermally decomposablematerial in at least one of the slurry coats subsequent to the firstbefore it is hardened, and

(b) subsequently heating the shell above the decomposition temperatureof the material to eliminate the material and leave voids distributedthroughout alternate layers of the shell wall.

2. In the manufacture of a refractory mold wherein a multi-layered shellis formed by successively dip-coating and hardening slurries comprisinga binder and a fine refractory on a fusible pattern at least seven timesand thereafter eliminating the pattern, a method of thermally insulatingthe shell which comprises (a) embedding an even distribution of aparticulate thermally decomposable material in alternate dipcoatssubsequent to the first two and excluding the last before each ishardened, and

(b) heating the shell after the pattern is eliminated above thedecomposition temperature of the material to eliminate the material andleave air-filled voids distributed in alternate layers throughout theshell wall.

3. A method according to claim 2 wherein the organic particulatematerial is either plastic, wood chips, cork, sawdust or corn chips.

4. In the manufacture of a refractory mold wherein a multi-layered shellis formed by successively coating and hardening slurries comprising abinder and a fine refractory on a pattern and thereafter eliminating thepattern, a method of thermally insulating the shell which comprises (a)distributing a particulate thermally decomposable material over thesurface of at least one of the slurry coats subsequent to the firstbefore it is hardened, and

(b) subsequently heating the shell above the decomposition temperatureof the material to eliminate the material and leave voids distributedthroughout the shell wall.

5. In the manufacture of a refractory mold wherein a multi-layered shellis formed by successively dip-coating and hardening slurries comprisinga binder and a fine refractory on a fusible pattern at least seven timesand thereafter eliminating the pattern, a method of thermally insulatingthe shell which comprises (a) distributing an organic particulatethermally decomposable material over the surface of alternate slurrydip-coats subsequent to at least the first two and excluding the lastbefore each is hardened, and

(b) heating the shell after the pattern is eliminated above thedecomposition temperature of the material to eliminate the material andleave air-filled voids distributed in alternate layers throughout theshell Wall.

6. A method according to claim 5 wherein the organic particulatethermally decomposable material is wood chips.

7. A method according to claim 5 wherein the organic particulatematerial is either plastic, wood chips, cork, sawdust or corn chips.

8. In the manufacture of a refractory mold wherein a multi-layered shellis formed by successively coating and hardening slurries comprising abinder and a fine refractory on a pattern and thereafter eliminating thepattern, a method of thermally insulating the shell which comprises (a)dispersing a particulate thermally decomposable material throughout theslurries used for at least one of the coats subsequent to the first sothat the material is embedded and evenly distributed in that coat beforeit is hardened, and I (b) subsequently heating the shell above thedecomposition temperature of the material to eliminate the material andleave voids distributed throughout alternate layers of the shell wall.

9. In the manufacture of a refractory mold wherein a multi-layered shellis formed by successively dip-coating and hardening slurries comprisinga binder and a fine refractory ona fusible pattern at least seven timesand thereafter eliminating the pattern, a method of thermally insulatingthe shell which comprises (a) dispersing an organic particulatethermally decomposable material throughout the slurries used foralternate dip-coats subsequent to at least the first three and excludingthe last so that the material is embedded and evenly distributed inthose alternate dip-coats before each is hardened, and

(b) heating the shell after the pattern is eliminated above thedecomposition temperature of the material to eliminate the material andleave air-filled voids distributed in alternate layers throughout theshell wall.

10. A method according to claim 9 wherein the organic particulatematerial is either plastic, cork, sawdust or corn chips.

11. In a thermally insulated refractory shell mold comprising aplurality of layers of bonded refractory material, the improvement whichcomprises (a) one or more alternate layers excluding the inner layerdefining at least part of a multiplicity of voids which reduce thecooling rate of the mold,

(b) at least the inner layer being free of such voids.

12. In a thermally insulated refractory shell mold comprising at leastseven layers of bonded refractory materail, the improvement whichcomprises (a) a plurality of alternate layers excluding the inner layerdefining at least part of a multiplicity of evenly distributedair-filled voids which reduce the cooling rate of the mold,

(b) the remainder of the layers being free of such voids.

13. A shell mold according to claim 12 wherein substantially every voidis defined by respective adjoining layers.

14. A shell mold according to claim 12 wherein substantially every voidis defined within one of the respective layers.

15. In a thermally insulated refractory shell mold comprising at leastseven layers of bonded refractory material, the improvement whichcomprises (a) a plurality of alternate adjoining layers, excluding atleast the inner two layers, defining between them a multiplicity ofevenly distributed air-filled voids which reduce the cooling rate of themold.

(b) I216 remainder of the layers being free of such 16. In a thermallyinsulated refractory shell mold comprising at least seven layers ofbonded refractory material, the improvement which comprises (a) aplurality of alternate layers, excluding at least the inner three layersand the outermost layer, each defining a multiplicity of evenlydistributed air-filled voids which reduce the cooling rate of the mold,

(b) the remainder of the layers being free of such voids.

17. In the manufacture of a refractory mold wherein a multi-layeredshell is formed by successively coating and hardening slurriescomprising a binder and a fine refractory on a pattern and thereaftereliminating the pattern, a method of providing a multiplicity of voidsthrough the shell which comprises:

(a) distributing a particulate thermally decomposable material over thesurface of at least one of the 7 slurry coats subsequent to the firstbefore it is hardened, and

(b) subsequently heating the shell above the decomposition temperatureof the material to eliminate the material and leave voids distributedthroughout the shell wall.

References Cited UNITED STATES PATENTS 2,948,032 8/1960 Reuter 164-26 8FOREIGN PATENTS 183,515 10/1955 Austria. 903,502 8/ 1962 Great Britain.302,619 1/ 1955 SWitZerland.

J. SPENCER OVERHOLSER, Primary Examiner.

E. MAR, Assistant Examiner.

11. IN A THERMALLY INSULATED REFRACTORY SHELL MOLD COMPRISING APLURALITY OF LAYERS OF BONDED REFRACTORY MATERIAL, THE IMPROVEMENT WHICHCOMPRISES (A) ONE OR MORE ALTERNATE LAYERS EXCLUDING THE INNER LAYERDEFINING AT LEAST PART F A MULTIPLICITY OF VOIDS WHICH REDUCE THECOOLING RATE OF THE MOLD, (B) AT LEAST THE INNER LAYER BEING FREE OFSUCH VOIDS.
 17. IN THE MANUFACTURE OF A REFRACTORY MOLD WHEREIN AMULTI-LAYERED SHELL IS FORMED BY SUCCESSIVELY COATING AND HARDENINGSLURRIES COMPRISING A BINDER AND A FINE REFRACTORY ON A PATTERN ANDTHEREAFTER ELIMINATING THE PATTERN, A METHOD OF PROVIDING A MULTIPLICITYOF VOIDS THROUGH THE SHELL WHICH COMPRISES: (A) DISTRIBUTING APARTICULATE THERMALLY DECOMPOSABLE MATERIAL OVER THE SURFACE OF AT LEASTONE OF THE SLURRY COATS SUBSEQUENT TO THE FIRST BEFORE IT IS HARDENED,AND (B) SUBSEQUENTLY HEATING THE SHELL ABOVE THE DECOMPOSITIONTEMPERATURE OF THE MATERIAL TO ELIMINATE THE MATERIAL AND LEAVE VOIDSDISTRIBUTED THROUGHOUT THE SHELL WALL.